Filters are circuits which may be used in communication systems to compensate for disturbances such as e.g. interference, etc. caused by the nature of the transmission media between sender and receiver. Filters remove the unwanted communication signal components and/or enhance the wanted communication signal components.
Radio Frequency (RF) filters and Microwave filters represent a class of filters, designed to operate on signals in the Megahertz to Gigahertz frequency ranges. This frequency range is the range used by most broadcast radio, television and wireless communication systems such as e.g. cellular communication systems, Wi-Fi, WiMax, LTE, etc. Thus most wireless communication devices will include some kind of filtering device performing filtering on the signals transmitted and/or received. Further are filters located in the radio interface communication nodes such as e.g. the radio antenna of the broadcast radio system, the TV broadcasting antenna of the television system and the radio base station of the cellular telephone system. Such filters are commonly used as building blocks for duplexers and diplexers to combine or separate multiple frequency bands.
Today two technologies predominates the radio base station front end filters. These filters, coaxial filters and ceramic filters each consists of a number of resonators, coupled together providing a proper transfer of wanted signals and rejection of unwanted signals.
A driving force within the development of filters today is the issue of size. The smaller the filters are, the smaller may the electronic devices, the filters are installed in, be made. This reduces the required space of the equipment for storage, shipment and installation at the customer's site. Thus it is desirable to be able to produce as small filters as possible with sufficient performance.
The performance of filters may be measured by their Quality or Q factor. A filter is said to have a high Q factor if the filter is capable of selecting or rejecting a range of frequencies that is narrow in comparison to the centre frequency. The Q factor represents a relationship between a stored and dissipated energy in a resonant circuit. The Q factor may be defined as the ratio of centre frequency divided by 3 dB bandwidth. The pass band loss of a filter is inversely proportional to unloaded Q.
The classical coaxial resonators with metal center conductor provide normal performance, such as e.g. a Q factor of 2500 with the cavity volume of 22*22*22 mm3 or a Q factor of 4300 with the cavity volume of 37*37*37 mm3 at 2 GHz frequency, and low manufacturing cost. These classical coaxial resonators are fairly scale able in size, such as e.g. coaxial resonator lengths from 15 mm to 100 mm depending on the frequency used. The coaxial resonators may make use of high-dielectric constant materials to reduce their overall size and thus enable the scalability. One disadvantage with ordinary coaxial resonators may be the limited power handling capability, which is caused by the small gap between the resonator and the tuning element.
The ceramic resonators, such as e.g. ceramic Transverse Electric (TE) TE01d single mode resonators, are used for high performance, such as e.g. a Q factor of 10000 and above. Ceramic resonators provide higher performance compared to classical coaxial resonators or waveguides with a maximum Q factor being less than 10000 at 2 GHz frequency.
Ceramic resonators are made of high-stability piezoelectric ceramics, generally lead zirconium titanate (PZT) which functions as a mechanical resonator. Ceramic resonators for TE and TM mode are made of a material compound of e.g. oxygen (O), barium (Ba), titanium (Ti), zinc (Zn), neodymium (Nd), and lanthanum (La). The TE01d single mode ceramic resonators require rather large cavities. At 1.9 GHz frequency the Q factor is about 3200 for a coaxial resonator when cavity is about 30*30 mm (height*diameter). The size of the TE01d single mode ceramic puck resonator is about 27.5*10 mm (height*diameter) in the same cavity as above. Smaller cavity size with TE01d mode at 1.9 GHz frequency is not possible, because it is then necessary to increase the puck diameter.
Furthermore a few manufacturers have used dielectric resonators such as e.g. Transverse Magnetic (TM) single mode resonators, as radio base station front end filters. TM resonators enable considerable size reduction compared to metal resonators. The Japanese patent application JP0310802 A, published May, 9, 1991, presents such a TM single mode resonator which facilitating size reduction without loss of performance relative to a metal coaxial resonator, the metal coaxial resonator having a coaxial metal rod which has the same resonant frequency as the ceramic TM single mode resonator. A typical TM single mode resonator saves 20-50% volume, depending on the resonant frequency and dielectric constant of the ceramic, compared to a coaxial metal resonator of the same unloaded Quality factor.
Other technologies which also have been used for radio base station front end filters are very complex shaped TM dual mode resonators and TM triple mode resonators. The Japanese patent application JP05048305 published Feb. 26, 1993, introduces such a small sized, light weight and inexpensive band rejection filter using a TM dual mode dielectric resonator. The size reduction with this technology is about 30-80% compared to a coaxial metal resonator of the same unloaded Quality factor and of the same resonant frequency.
The application of TM mode is when both resonator ends are grounded. A commonly used method for grounding both resonator ends is e.g. soldering the dielectric rods directly to the filter housing and filter lid. A problem with the existing solution, using soldering to attach the TM mode dielectric rods to the filter housing and/or the lid, is that once the dielectric rod have been assembled and soldered, the dielectric rod cannot be replaced. To replace one single dielectric rod in a filter, at least one end of all the other soldered dielectric rods in the filter must be de-soldered, such as e.g. de-soldered from the lid side. In practice, this is however not possible, due to the fact that the conductive plating material, such as e.g. the silver plating, at the dielectric rod ends will only be good for one soldering operation, thus the dielectric rod is not replaceable.
After the filter has been assembled by grounding the dielectric rods, the filter is frequency tuned. The filter may be frequency tuned by removing material from the dielectric rods. However, frequency tuning performed by removing material is irreversible, i.e. the frequency tuning can only be performed in one way. This involves a considerable risk that too much material is removed which makes the dielectric rods and the whole ceramic filter accordingly of less use and most probably even useless. Secondly there is a considerable risk that the dielectric rods are damaged. Today it is not possible to add material that has been removed, or repair a dielectric rod that has been damaged. The result of this is a potentially very high scrap cost since the whole filter has to be scrapped if one single frequency tuning operation fails.
Another way of frequency tuning the filter is by inserting a tuning screw into a hollow in the dielectric rod as presented by the U.S. Pat. No. 6,535,086 B1, issued Mar. 18, 2003. This method is reversible since the tuning screw easily may be screwed in and unscrewed. However a disadvantage of this method is that the hollow and the tuning screw will decrease the filter performance, i.e. lower the Q factor, and also reduce the power handling capability. This results in that the size reduction is smaller using this method of frequency tuning than with the method of removing material from the dielectric rod to tune the frequency.
A further disadvantage of the existing solutions of grounding the dielectric rod ends by soldering is that it is difficult to get a repeatable soldering process for a complete filter consisting of several dielectric rods when soldering directly to the filter housing and the filter lid. This is because of the product mass of the filter housing and filter cover being high which causes the filter housing, filter cover and ceramics to heat up slowly which delays the soldering considerably. This may further have a negative effect on the long term solder joint reliability. When the whole filter assembly is heated, it is also required that all components inside the product can withstand the soldering temperature; this will limit the choice of material, such as e.g. plastics, and possibly also increase the material cost. A high mass product is also difficult to handle after the soldering operation because of the latent heat in the filter housing, lid and ceramics, thus the cool down process has to be long, which increases the manufacturing lead time and cost. Further the multiple solder joint orientations, such as e.g. two directions in the case of a TM single mode filter or four in the case of a TM dual mode filter, complicates the soldering process. Moreover, when soldering directly to the filter housing and lid it is necessary to have tight mechanical tolerances, and that is also cost driving.